The present invention relates generally to medical diagnostic imaging systems, and in particular relates to a system and method for the elimination of the effects of saturated pixels in solid state x-ray detector images.
X-ray imaging has long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial and abdominal images that often include information necessary for a physician to make an accurate diagnosis. X-ray imaging systems typically include an x-ray source and an x-ray detector. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against the x-ray sensor as an x-ray technologist positions the x-ray detector and the x-ray source at an appropriate height. X-rays produced by the source travel through the patient's chest, and the x-ray detector then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system obtains the detected x-ray energy from the x-ray detector and prepares a corresponding diagnostic image on a display.
In addition, x-ray images may be used for many other purposes. For instance, internal defects in a target object may be detected by x-ray images. Additionally, changes in internal structure or alignment of a target object may be determined from an examination of an x-ray image. Furthermore, the x-ray image may show the presence or absence of objects in the target. The information gained from x-ray imaging has applications in many fields other than medicine, including, for example, manufacturing.
The x-ray detector may be a conventional screen/film configuration, in which the screen converts the x-rays to light that exposes the film. The x-ray detector may also be a solid state digital image detector. Digital detectors afford a significantly greater dynamic range than conventional screen/film configurations.
FIG. 1 illustrates an exemplary solid state x-ray detector 100. The solid state x-ray detector 100 includes an array of picture elements (pixels 150), an x-ray conversion layer 120 and readout electronics 140.
The array of pixels includes a plurality of pixels 150. Each pixel 150 includes a switching element (“switch”) and a light detector. Typically, the pixel 150 switch is a field effect transistor (“FET”) and the light detector is a photodiode. The pixels 150 are generally composed of amorphous silicon. The x-ray conversion layer 120 may be a layer of CsI deposited on the pixel array.
The array of pixels 150 may be arranged in columns 170 and rows 160. Generally, the pixels 150 included in a row 160 are connected to a data line via the pixel 150 switch. A data line may be a conductor common to all pixels 150 in a given row 160. Each data line is connected to readout electronics 140. The readout electronics 140 measure the amount of charging current supplied to pixels 150 connected to a data line.
Generally, the pixels 150 included in a column 170 are connected to a scan line via control terminals of the pixel 150 switches. The scan line may be common to all pixels 150 in a given column 170. A scan line may be asserted to allow charging current to flow from the readout electronics 140 to the pixels 150 connected to the scan line. The various scan lines may be connected to scan drivers. The scan drivers may be included in a data acquisition system 190. The scan drivers activate a given scan line by asserting the scan line.
Prior to x-ray exposure, the detector 100 must be initialized. Initialization of the detector 100 occurs by charging each photodiode in the various pixels 150 with a reverse bias voltage to a known voltage. This is naturally accomplished in the course of scanning the detector 100. The readout electronics 140 provide a constant voltage on each of the data lines. As the data acquisition system 190 asserts each scan line (via a scan driver, as described above), the FET in each pixel 150 connected to that scan line conducts electrical charge from the data line connected to that pixel 150. In this way, the photodiode for each pixel 150 of the scan line may be recharged to the potential difference between the data line and a common electrode. Since the common electrode generally has a negative potential with respect to the data line, the photodiode is generally reversed biased, and therefore does not conduct charge other than leakage current. Therefore, the photodiode may then simply store the charge provided via the data line.
In operation, the photodiodes in the pixels 150 measure an amount of x-ray exposure. When incident x-ray flux strikes the x-ray conversion layer 120, the layer 120 converts the x-ray flux into light. The amount of light converted by the layer 120 is generally proportional to the intensity of the incident x-ray flux. The light then strikes the photodiodes in the pixels 150. Each photodiode is initially charged with a known amount of reverse bias voltage. When the light strikes the photodiodes, the photodiodes conduct and an amount of electric charge is discharged from the initially charged amount of reverse bias voltage in the photodiode. That is, the incident light discharges some or all of the reverse bias voltage initially charged in the photodiodes. The amount of discharged voltage in the photodiodes is generally proportional to the intensity of incident light, which is generally proportional to the intensity of the incident x-ray flux. Therefore, the amount of discharged voltage in photodiodes is generally proportional to the intensity of incident x-rays.
After the conclusion of the exposure, the voltage on the photodiode is restored to the initial voltage. The amount of charge required to restore the photodiode to the initial voltage is measured as restoring charge. The restoring charge is therefore a measurement of the x-ray flux intensity integrated by the pixel 150 during the length of the exposure.
The detector 100 is scanned by the readout electronics 140 in a manner similar to the structure of the pixel array. The detector 100 is scanned on a column-by-column basis for the various columns 170 of pixels 150 in the pixel array. In operation, a scan line is asserted by a scan driver in the data acquisition system 190. As described above, each of the pixels 150 along the scan line is connected to a separate data line. When the scan line is asserted, the gates of the FETs in the pixels 150 connected to the scan line conduct. The data line then conducts the charge to the photodiode that has been discharged due to x-ray exposure. As each scan line is asserted in turn, the initial voltage is restored to all of the photodiodes of the pixels 150 in the scan line simultaneously by the readout electronics 140 over the individual data lines. The amount of restoring charge for each pixel 150 is provided by the readout electronics 140 through the data lines.
The scan drivers may assert the individual scan lines in a given sequence. Therefore, the pixel 150 array may be read-out, or scanned, by selectively asserting a scan line, followed by asserting a different scan line, and so on, until all of the desired pixels 150 have been read-out by asserting the associated scan lines.
In any imaging system, x-ray or otherwise, image quality is important. X-ray imaging systems utilizing digital or solid state image detectors (“digital x-ray systems”) experience certain electrical phenomena that may cause imaging difficulties. Imaging difficulties may be caused by effects such as electronic current leakage from imaging system circuitry, x-ray detectors, and the like.
At least one region of interest (“ROI”) may be identified in an exposure of the detector 100. Generally, the ROI is an area of the detector 100 where the object being examined impedes the path of incident x-rays to the detector 100. The ROI is an area of the detector 100 where the detector 100 may receive a lesser intensity of x-ray flux intensity. Therefore, as described above, less incident x-ray flux may be converted into light by the scintillator 120, causing less voltage to be discharged in the various photodiodes of the pixels 150. Pixels 150 located within the ROI are therefore unsaturated pixels 150.
Pixels 150 located outside the ROI may be exposed to raw x-ray beams and therefore may become completely discharged of the initial voltage. Pixels 150 that become completely discharged of the initial voltage are saturated pixels 150. Saturated pixels 150 place a large amount of electronic stress on the photodiode FETs. In addition, for detectors 100 that use a scintillator and a photodiode, optical light emitted from the scintillator may be absorbed in the active area (particularly the FETs) causing an increase in this leakage current.
Because the exact location of the various saturated pixels 150 may be unknown prior to the scanning of the detector 100, each saturated pixel 150 on a data line may add an indeterminable amount of leakage current to the signal of each pixel 150 scanned prior to the saturated pixel 150 in the data line. If the saturated pixels 150 on a data line are scanned after unsaturated pixels 150 in the ROI on the same data line, the leakage current from the saturated pixels 150 may add to and corrupt the signal obtained from the unsaturated pixels 150. The reading of an unsaturated pixel 150 before a saturated pixel 150 in a data line therefore may cause the signal read from the unsaturated pixel 150 to become corrupted.
When the signal read from the unsaturated pixel 150 becomes corrupted, the x-ray intensity experienced by the unsaturated pixel 150 is inaccurate in the resultant x-ray image. For example, an unsaturated pixel 150 with a corrupted signal (caused by the reading of the unsaturated pixel 150 before a saturated pixel 150) may appear to have been exposed to a greater x-ray intensity than the actual x-ray intensity exposed to the unsaturated pixel 150. This, in turn causes the unsaturated pixel 150 to appear lighter in the resultant x-ray image. In addition, leakage current from the saturated pixels 150 may cause edges of an object to become blurred in the resultant x-ray image. As such, leakage current from the saturated pixels 150 may cause degradation in x-ray image accuracy, contrast and quality.
Many solid state x-ray detectors currently use a “split data line” design. FIG. 2 illustrates an exemplary split data line detector 200. The split line detector 200 includes several data lines 210, a data line split 230 and several scan lines 250. As described above, each data line 210 includes a plurality of pixels 150. Each data line 210 does not extend across the full vertical length of the detector 200. The various data lines 210 extend from opposite edges of the detector 200 to the center of the detector 200. The data lines 210 in the two halves of the detector 200 meet at a data line split 230. The scan lines 250 also include a plurality of pixels 150. Each scan line 250 extends across the full length of the detector 200.
In operation, an object is placed above the detector 200 and is exposed to x-ray flux. As described above, pixels 150 in the detector 200 located underneath the object are located in an ROI 220. The pixels 150 in the ROI 220 are generally unsaturated pixels 240. Pixels 150 outside of the ROI 220 receive raw x-ray beams and are therefore generally saturated pixels 260. In this way, a pixel 150 may be either an unsaturated pixel 240 or a saturated pixel 260, depending on the amount of stored charge that is discharged in the pixel 150.
When the split data line detector 200 is scanned, a scan line 250 along the top of the detector 200 and a scan line 250 across the bottom of the detector 200 are activated simultaneously. The simultaneous activation of the scan lines 250 causes all the pixels 150 along those two respective scan lines 250 in the top and the bottom of the detector 200 to be read simultaneously. After the top and bottom rows of pixels 150 are read, the respective scan lines 250 are deactivated, and the second-to-top scan line 250 and the second-to-bottom scan line 250 are activated. The sequential activation, scanning and deactivation of adjacent scan lines 250 continues from the scan lines 250 at the opposite edges of the detector 200 and ends at the data line split 230.
However, as described above, leakage current from saturated pixels 260 read after unsaturated pixels 240 may corrupt the resultant signal. According to the direction of the progression of scan line 250 activation, scanning and deactivation illustrated in FIG. 2, the split data line detector 200 reads many saturated pixels 260 after the unsaturated pixels 240 of the ROI 220. Thus, the signal from many of the unsaturated pixels 240 of the ROI 220 may become corrupted from the leakage current of the saturated pixels 260.
Therefore, a need exists for a method and system for reading the signals from a solid state x-ray detector to reduce the effects of saturated pixels on the signals from unsaturated pixels.